Hammer module for high-speed line printer

ABSTRACT

For line printers of the type wherein a web of paper is imprinted by impact by a line of hammers on one side of the sheet which drives the paper against selected ones of type faces driven rapidly and continuously past the printing line behind the web, a simplified hammer module is disclosed. The module embodies socalled free-flight hammers which are thrown against the web by electromagnetic actuators. The hammers are guided by simple rounded grooves in which they rest. Simple mild steel actuator arms are shown to have necessary mechanical an d magnetic properties to link electromagnets and hammers. Electromagnets are arranged in two rows whereby alternate hammers are thrown by straight and offset actuator arms, achieving a module for 80 hammers with pica spacing all of which is below the print line for quick reading accessibility.

United States Patent {72] Inventors Byron D. Smith Boxiord, Mm; RobertHermann, Rye, N.l-1. [21] Appl. No 849,971 [221 Filed Aug. 14, 1969 [45]Patented June 15, 1971 [73] Assignee Syner-Data,lnc.

Beverly, Mass.

[54] HAMMER MODULE FOR HIGH-SPEED LINE PRINTER 10 Claims, 2 DrawingFigs. [52] US. Cl 101/93 [511 1nt.Cl B4lj 9/10, B41 j 9/38 [50] Field ofSearch 101/93; 317/155.5,123 [56] References Cited UNITED STATES PATENTS2,940,385 6/1960 House 101/93 3,145,650 8/1964 Wright 101/93 3,266,4188/1966 Russo 101/93 Primary Examiner-William B. Penn Attorney-JackLarsen ABSTRACT: For line printers of the type wherein a web of paper isimprinted by impact by a line of hammers on one side of the sheet whichdrives the paper against selected ones of type faces driven rapidly andcontinuously past the printing line behind the web, a simplified hammermodule is disclosed. The module embodies so-called free-flight hammerswhich are thrown against the web by electromagnetic actuators. Thehammers are guided by simple rounded grooves in which they rest. Simplemild steel actuator arms are shown to have necessary mechanical an dmagnetic properties to link electromagnets and hammers. Electromagnetsare arranged in two rows whereby alternate hammers are thrown bystraight and offset actuator arms, achieving a module for 80 hammerswith pica spacing all of which is below the print. line for quickreading accessibility.

PATENTEDJUHTSIQYI 3.5847574 sum 1 BF 2 INVENTORS BYRON D. SMHTH ROBERTHERMANN BY (W ATTORNEY PATENTEDJUHISIQH 3584.574

1N VIiN'I'ORS YQM D. SMBTH OBERT MERMANM ATTONEY HAMMER MODULE FORHIGH-SPEED LINE PRINTER This invention, generally, relates to high speedprinter hammer modules of the type used in printers used withdataprocessing equipment, and more particularly it concerns a printhammer module particularly adapted to operate at the informationcapacity of an ordinary telephone line.

In high speed printers of the type aforementioned, a moveable carriersupporting a plurality of type faces is moved continuously past a printline at a high rate of speed. A pring medium, such as a web of paper, isfed incrementally, step by step past the print line, and an alignedarray of print hammers is arranged for impact printing along the printline, one hammer being provided for each print position. The resultingprinted copy is arranged in horizontal rows and vertical columns, onecolumn for each hammer and print position, The type carrier may be adrum rotating on a horizontal axis with a complete font of charactersarranged in a ring around the drum for each printing position. For picatypewriter characters, columns are spaced 10 to the inch, and a drumabout 3 inches in diameter will accommodate 64 characters of an uppercase only font, while somewhat less than a 6-inch diameter mayaccommodate a 128 character font of upper and lower case characters.

Alternatively, the type font may be carried on a belt or large wheelwhich traverses horizontally along the print line. Traversing eitherway, vertically or horizontally, all of the characters must pass eachprint position in each cycle time of the printer. Printers of this typemay operate at a speed of 5 to lines per second and higher. Given 80pica characters per standard line, and 64 characters to a minimum font,the minimum speed of the type relative to the web is about 30 inches persecond. If smear is to be limited to one one-hundredth of an inch, thenimpact between hammers and the moving type faces must last for only afraction of a millisecond. To minimize smearing of the copy, variousexpedients have been adopted to accelerate and control the motion of thehammers. A further alternate approach is represented by U.S. Pat. No.3,285,164 to Malavazos wherein a horizontally driven chain carries, notindividual type faces, but print wheels, each having a differentcharacter repeated 6 times around the wheel which is driven by anendless rack and pinions to have rolling contact at the points where theimpressions are made. Print hammer modules embodying the presentinvention may be employed in place of hammer modules of the prior-art inconjunction with each of these various kinds of machines.

Printers of the types described above operating at the higher speedrange of 10 lines per second and more, and 128 characters to the font,represents an information flow rate of 5,600 bits/second or more. Thishigh information flow rate is required to approach the power of a moderndigital computer to generate date; but is several times the informationhandling capacity of ordinary telephone lines and its terminalequipment.

Because of the complexity required to achieve such high data rates, lineprinters of the type described are very expensive. Of considerably lowercost are various teletypewriters and tape-controlled typewriters whichare capable of speeds up to the order of 40 characters per second,compared to 800 characters per second of the line printers. These lattermachines are not cheap either. Moreover, they fall far short of matchingthe information capacity of a minimum quality telephone circuit.

The principal object of the present invention is to provide a simple,inexpensive print-hammer module whereby to provide a line printer withcapacity to transcribe the maximum information rate of an ordinarytelephone line, yet to cost no more than much lower speed automaticelectric typewriters.

An essential feature of a line printer is its memory in which it acceptsa batch of information from a source, which may be a telephone line. Thememory is then read out as timed pulses to the several hammers duringone cycle of the type carrier. Meanwhile, the memory must continue toaccept the next batch of copy (which may amount to a line, or a fractionof a line) while the previous batch is being printed. This memory may beimplemented in various ways; but one implementation requires a sixorseven-bit shift register for each column of type. With the advent ofintegrated circuit shift-registers, and other advanced memorytechniques, the electronic memory is reduced to a minor portion ofthecost of the printer. Once the mold is made, plastic molded print rollersare relatively inexpensive. The principal remaining contributor to thecost of such a printer, and clearly the most critical portion,principally limiting performance is seen to be the print hammer module,the assembly of 20 to 160 (typically similar electromechanicalmechanisms, spaced only one-tenth of an inch apart, on the average,along the print .line.

Various types of hammers have been tried in such line printers,including hammers driven by hydraulic, pneumatic, and magnetic forces.In some mechanisms, the hammers are pivoted in the manner ofconventional typewriter keys, while in others, a so-called free-flighthammer is employed.

A hammer module of the type wherein the hammer faces are on the ends ofmagnetically operated actuator arms is disclosed in Potter US. Pat. No.3,349,696. Since it is often necessary or desirable to vary thethickness of paper or carbon and paper, the free-flight type hammer ispreferred wherein the hammer is struck or thrown by the actuator, thearmature can bottom before the hammer strikes, thus keeping efficiencyhigher than if adjustments were required to enable the armature tobottom for varying paper thicknesses.

The free-flight hammers must be guided in the flight to insure that theywill strike the rotating drum at precisely the correct places on thedrum periphery. Also, the hammers must be returned to their restposition for the next print cycle.

US. Pat. No. 3,145,650to Gerard C. Wright discloses a so phisticatedarrangement for such guidance, to overcome difficulties in the priorartwherein each individual hammer is in a supporting channel or guide sothat upon the impact of the actuator the hammer moves forward to effecta printing stroke. Upon the completion of this stroke, the hammerreturns to its original or rest position within the guide by springmeans. Due, however to varying frictional forces, dust, or lint withinthe guides, the flight of the hammer, either in its forward motiontoward the drum or in its spring actuated motion back to its restposition, is often erratic. Such erratic action results in variations inflight time and consequently irregularity of the print line. Cleaningsuch channels is a time-consuming and expensive procedure, and thefabrication of the channels requires very accurate machining to insureproper fit.

It is an object of the present invention to provide a simplified hammerguide arrangement which avoids the abovementioned shortcomings of theprior art with a compact and simple hammer guide arrangement which isinexpensive to manufacture and easy to clean and more tolerant of lintand grit.

It is a further object of the invention to provide a simple and reliablestructure to hold and locate hammers and actuators in correct alignmentone with other, and a further objective to provide simple spring meansto return hammers and actuators to their rest positions.

It is a further object of this invention to provide an improvedarrangement of actuators whereby they may be closely spaced, yet occupyonly one side of the print line so that the emerging copy may be readnot more than I second after printing.

A feature of the invention by which the above objects are attained is aone-piece guide block for hammers and actuators. Another feature is aone-piece spring comb by which a group of hammers and actuators arereturned to their rest positions.

A further feature of the invention is a simplified actuator arrangementproviding for alternate straight and offset actuators whereby to providespacing of one-fifth of an inch between coils on each of two solenoidassemblies, one for the straight actuators and one for the offsetactuators, while having only one-tenth of an inch on centers betweenactuator throwing arms and hammers.

Other features of the invention will appear in the description of theinvention below with reference to the accompanying drawings in which:

FIG. 1 is a view, partly in section, illustrating the relative situationof the printing drum, hammers, actuators, mounting means for the hammersand actuators, etc. and

FIG. 2 is a cutaway perspective view of the preferred embodiment of theinvention.

As shown in FIG. 1 a drum is provided which rotates to carry the typefaces 12 upward past the hammers 14. In the preferred embodiment thestandard ASCll (63 character plus blank) font is employed. Each of thehammers 14 is normally held in its rest position away from the drum by areturn spring leaf 16 which engages a recess in the form of a groove 17in the hammer. Alternately the hammers are engaged at their tail ends 18by straight actuators 20 and offset actuators 22. Straight and offsetactuators have identical throwing arms 26 which are thin pieces ofordinary cold-rolled steel, for simplicity cut from flat stock 0.032inches thick, thick enough not to buckle, yet thin enough to fit in theavailable space, tapered toward the upper end from a maximum depth ofthe cantilever beam of 0.3 l 2 inches at the pivot pin 30 to a minimumdepth of one-sixteenth inch. The actuators, hammers, and return springsare all mounted on a hammer and actuator guide block 40. This block,which preferably is formed from a single piece of metal or plastic has amounting flange 41 (shown in FIG. 2) at each end from which it issupported in parallel relationship to the drum 10. A hammer mountingchannel 42 is formed at the top 43 of the block 40 which is just over 8inches wide, and somewhat over one-eighth inch deep. At the bottom ofthis channel there are 80 near semicylindrical grooves 44 spacedone-tenth inch on centers. A set of the hammers 14 may be dropped intothis channel and tend to be evenly spaced by these grooves. Hammers areonly slightly less than a full column width (preferably 0.095 inches).They are simple bars, the height being determined by the height of theletters in the type font. The head end of the hammer bar provides thestriking face which must be large enough and proportioned to cover allof the characters in the font. The top is flat, to bear against thecover 46 which retains the set of hammers in the channel 42. The bottomsof the hammers are curved hemicylindrically on a 0.0475 inch radius,half the hammer width. A slot 48 extends the width of the channel 42intercepting the grooves 44 and cutting into portions carryingrespectively the head and tail ends of the hammers. The slot 48 providesa place for lint and grit, which may enter the channel 42 to drop downout of contact with the hammers 14.

The block 40 has a face 50 facing the impression sheet 52, and extendsdownwardly about 2 inches and parallel to the drum axis. Generallyparallel to the face 50 is the back 54.

At the bottom of the block and extending outward from the back is a foot60 having large toes (not shown) on each end and 79 small toes 62separating and guiding the 80 actuators. A notch 64 on the bottom ofeach of the toes provides an opening through which the pivot pin 30passes and is located. The pivot pin is retained in place by threeretainer fingers 68, which are fastened to the foot 60, one at each endand one in the middle.

From the pivot pin 30, an armature arm 90s extends downward for each ofthe straight actuators, while the armature arm 900 extends generallyhorizontally out from the foot 60. Each of these arms carries anarmature piece 92 of mild steel or other material of high magneticpermeability. Opposing each of these armature pieces is anelectromagnet. Two sets of cores 96s and 960 are provided. In each set,the electromagnets are spaced on centers by twice the column width, andthereby one set of magnets may be placed in opposition to the straightarmature arms, and one set of magnets may be placed in opposition to theoffset armature arms. Since the force of the magnets is limited by poleface area, this arrangement allows for a more conservative magnetdesign, using simple mild steel cores, ordinary magnets would be limitedto one column width. This kind of staggered arrangement has been favoredin the prior art as represented by the patent to Wright; but in theprior art the actuators have been distributed above and below the printline. In arrangements such as shown in Wright, the print line is hiddenbehind the hammer modules with the result that there is a delay after aline is written before it can be read.

-The cores 960 are of a U-shaped contour (so-called C cores) withwindings 97 on two long legs and with retention notches 100. The coresmay be laminated or a single piece of 013 gage, (0.0897 inches thick)mild steel. Cores are preferably 1.650 inches long, by 0.781 inches widewith a slot between the legs about 0. 1 6 inches wide and 1.16 incheslong.

The cores are assembled to a channel 102 having end fittings 104 bymeans of a casting resin 105, preferably an epoxy formulation. Theassembly of cores, and mounting channel comprise a core assembly bar.The bar 1060 containing the cores 960 and the bar 105s containing thecores 96s for the straight actuators may be identical andinterchangeable, turning one end for end to replace the other.

A pair of plastic spools 107 and 108 carry the windings 97 for eachcore. Preferably these spools are wound in series on a mandrel, thenslipped over the core legs. A total of 300 turns of 030 enamelled magnetwire in three layers is appropriated. Molded integrally with the spool107 are a pair of contacts 109 to which the ends of the winding areattached, leaving the pins projecting outwardly from the core assemblybars in 0.10 inch spacings. Individual windings may be replaced ifnecessary without destruction of a core assembly bar.

Two identical end frames 110 are provided which support the ends of theguide block 40 which carries the hammers l4 and the actuators 96s and960. To maintain accurate relationships between the hammers and theprint roller, and between the actuators and their respective coreassembly bars, the roller and core assembly bars are carried on the sameend frames 110. The bearings 112 for the print roller are retained bypillow blocks 114.

A very simple arrangement of return springs is provided. A spring comb116 is provided. Fastened by screws to a spring boss 118 extendingacross the block 40 between the grooves 44 and the foot 60, the comb hasas long teeth extending upward the leaves 16 to engage each of thegrooves 17 in the tail ends of the hammers 14 and serve as hammer returnsprings. Actuator return springs 128 are provided by shorter, andtherefore stiffer, teeth extending downward to engage spring returnabutments 130 at the front of each throwing arm about three-eighth inchfrom the pivot pin.

The rest position of the hammers 14 is against the actuator throwingarms, while, in turn, the rest position of the actuators is determinedby the contact of stop abutments 134s and 1340 at the ends of thestraight and offset armature arms respectively with stop bars 135 and136 respectively, fastened to the end frames.

Connections between the windings 106 and the external circuitry are byway of the connector pins 107, 108 (one of each per core) mounted to thespools 107. The pins, spaced 0.10 inches apart are engaged by connectors(not shown). One connector is provided for each 10 cores. Successivecharacters may follow rapidly in time as a given line is printed, thenprinting must be stopped while the paper is indexed to the next line.This can be accomplished in about 20 milliseconds, less than the timefor a quarter turn of the print drum. If each and every one of the 63characters in the font were to appear on each line, then at least 1%turn of the print drum would be required for each line of copy. If theline rate is synchronized with the drum rate, then, also at least 1%turn is required per line of copy. Fortunately the copy to be printeddoes not call upon all of the characters with equal frequency. Letters,numerals punctuation, and special symbols are grouped so that,ordinarily, one of these groups is not called for in a line, so

that paper may be indexed as the characters of the unused group pass theprint line. The memory logic may be arranged so that when all of thecharacters in a line are printed, this fact is indicated to trigger thepaper advance mechanism. Once a hammer has been thrown, it cannot becalled upon for a subsequent throw until at least the time required forpaper advance has elapsed. Thus, while the hammer must strike quickly(in the order of a millisecond), it may be relatively leisurely inreturning to its rest position.

The hammers 14 are returned to rest position primarily by reboundingfrom the elastic print drum l0. Assisting the rebound, the spring leaves17 insure that at the start of actuator action, the hammers are incontact with the actuators.

In asynchronous operation, printing of a line may begin at any positionof the print roller. Suitable circuitry, which may include aphotoelectric pickoff and apertured disc (not shown) rotating with theprint roller 10, provides electrical impulses timed to coincide with theprogression of the characters on the print roller past the print line. Aparticular time delay corresponds to each character. The shape andduration of the current pulse to the electromagnets is not critical; butfor timing the shorter the pulse, the better. if the pulses are tooshort, then voltages and currents become unacceptably high. Forsimplicity, a solid-state switch circuit closes for just 2 milliseconds.For a 63-character font at five printed lines per second this is onlyslightly less than the character time. Because of armature inertia andcore inductance, the current increases and the airgap in the magneticcircuit decreases during an actuator cycle until at the instant when thearmature bottoms in contact with the pole faces 144, 146, theinductance, current and force are maximized. Because of the rapidbuildup of force, there is some bending of the throwing arm at theinstantthe armature bottoms. Thereafter the tip of the throwing armcontinues to move forward as the bending stress is relieved, andtheresulting hammer speed continues to increases until the arm is relaxed,at which point, the hammer separates from the actuator and continues inrelatively free flight to strike the paper. Because of its inertia, thethrowing arm tends to follow through, then whip back assisted by thereturn spring leaf 128 and the termination of magnetizing current.

Depending upon the copy to be transcribed, any number of hammers fromzero up to all 80 may be called upon to strike at a given instant, theaverage number is small, but there is a substantial probability that aline of copy may contain a great many zeros; or a dashed or dotted linemay be called for. To avoid having such copy calling for simultaneousoperation of all 80 hammers, the print roller is commonly segmented into20-column sections, with the characters of the sections staggered onewith respect to the next. The power supply for the hammer electronicsmay then be made marginal for the case when 80 hammers are called for,with the probability negligibly small that more than 40 hammers would becalled for at a time. This problem does not arise with printers in whichthe type carrier is a chain or belt which carries the type faceshorizontally along the printing line.

We claim:

1. For printing a sheet with character positions arranged in regularlines and at least eight columns across said sheet a hammer module for ahigh-speed printer, of the kind wherein characters are selected from atype carrier circulating con tinuously behind said sheet carrying typefaces for each of the characters of a font to present at each characterposition on a print line different ones of said faces in successivecharacter times, so that by striking the paper from the front withhammers arranged in a line across said sheet, the blow of each hammerbeing timed to be coincident with the passing of a character position bythe desired character for that position, the several hammers beingvariously timed to effect the printing along said print line of apredetermined line ofcopy within a single circulation of said carrier,the combination characterized by a. a plurality of such hammers, one foreach said column, each of said hammers comprising a rigid bar having ahead-end striking face adapted to press upon impact against the typeface of the selected character, said hammer being of a width closelyapproaching, but not exceeding, the width of said column, having flat,vertical, and parallel sides, and a cylindrical bottom surface, wherebysaid hammers when situated side-by-side have freedom to slide oneagainst the next, but are restrained by interference each with the: nextagainst excessive tipping, each said hammer having a tail end adapted toabut the throwing arm ofa corresponding hammer actuator,

b. mounting means situated at the ends of said module,

c. an equal plurality of actuators,

d. a hammer and actuator guide block, said block having end partsattached to said mounting means and a central portion, said centralportion having a face, situated facing and generally parallel to saidsheet, parallel to said print line, a back generally parallel to saidface, a top, extending generally perpendicular to said face andsubstantially coplanar with said print line, the upper surface of saidtop being figured by a sequence of parallel rounded grooves extendingperpendicular to said sheet and spaced on centers by the width of saidcolumns to support and space said hammers, and said block having a footwith toes extending outward from said block below and generally parallelto said grooves and centered between them, parallel and vertical sidesof said toes providing guiding slots for said actuators, said toeshaving bearing openings to support a pivot pin for all said actuators,

e. a pivot pin passing through said openings,

each of said actuators comprising a piece of metal having uniformthickness to fit between said toes having a depth sufficient to besubstantially rigid in rotation about said pivot pin, having a throwingarrn portion tapering from said pivot pin to a thin end to abut the tailend of said hammers, passing between said toes in said slots andpivotally mounted on said pivot pin,

g. said actuators comprising a set of straight actuators and a set ofoffset actuators, said offset actuators being alternately installedbetween said straight actuators, each said straight actuator having astraight armature arm extending generally parallel to said sheet belowsaid pin as an extension of said throwing arm, having a soft-magneticarmature piece fixed to a side thereof,

h. each of said offset actuators having an offset armature extendinggenerally perpendicular to said sheet and perpendicular to said straightarmature arms, having a soft-magnetic armature piece fixed to a sidethereof,

. a first electromagnetic assembly comprising a plurality ofelectromagnets each having a core and a magnetizing winding on saidcore, said cores being assembled parallel, spaced apart on centers bytwice said column width and presenting a first working surface in whichpole faces of said cores are regularly arrayed,

j. a second electromagnet assembly substantially identical to said firstelectromagnet assembly having a second working face,

. said first electromagnet assembly being attached to said mountingmeans to situate said first working face in close opposing relationshipto said armature arms of said straight actuators, each one of said firstsurface pole faces being in operable opposition to a corresponding oneof said armature pieces, and

. said second electromagnet assembly being attached to said mountingmeans to situate said second working surface in close opposingrelationship to said offset armature arms, each one of said secondsurface pole faces being in operable opposition to a corresponding oneof said armature pieces.

2. A printing mechanism comprising a. a hammer module as defined byclaim 1 wherein said mounting means comprise a pair of end framesgenerally parallel and extending away from said sheet and downwardlyfrom said line of text to a machine frame and b. a type-carrying drumrotatably supported on said end frames and continuously rotatable behindsaid sheet containing for each of said columns a font of type arrangedin a ring around said drum.

3. A hammer module as defined by claim 1 wherein said metal is mildsteel.

4. A hammer module as defined by claim 3 in further combination with aflat spring piece cut to form a hammer return spring comb having teethin the form of leaf springs each adapted to engage at its tip the tailend ofa corresponding one of said hammers, the body of said piece beingfastened to said vback above said toes.

5. A printer comprising a. a hammer module as defined by claim 41wherein said mounting means comprise a pair of end frames generallyparallel and extending away from said sheet and downwardly from saidline of copy to a machine frame and b. a type-carrying drum rotatablysupported on said end frames and continuously rotatable behind saidsheet containing for each of said columns a font of type arranged in aring around said drum.

6. A hammer module as defined by claim 1 wherein said grooves aredivided by a slot cut into said block top parallel to said print lineinto front and back portions, in further combination with a spring piececut to form a hammer return spring comb having teeth in the formofspring leaves each said leaf adapted to engage at its tip the tail endof a corresponding one of said hammers, the body of said piece beingfastened to said back above said toes, and wherein each of said hammershas a recess at its tail end to engage one of said leaves.

7. A hammer module as defined by claim 6 wherein said metal is mildsteel.

8. A hammer module as claimed 1 in further combination with a straightstop bar maintained by said mounting means parallel to said print linein opposition to stop abutments on said straight actuators, whereby toestablish the rest position of said actuators.

9. A hammer module as defined by claim 8 wherein said metal is mildsteel.

10. A hammer module as defined by claim 9 in further combination with aspring piece cut to form a return spring comb having upward extendingteeth in the form of spring leaves each said leaf being adapted toengage a recess in the tail end of a corresponding one of said hammers,and having downward extending teeth situated to engage return springabutments on corresponding ones of said throwing arms, the body ofsaidpiece being fastened to said back above said toes.

1. For printing a sheet with character positions arranged in regularlines and at least eight columns across said sheet a hammer module for ahigh-speed printer, of the kind wherein characters are selected from atype carrier circulating continuously behind said sheet carrying typefaces for each of the characters of a font to present at each characterposition on a print line different ones of said faces in successivecharacter times, so that by striking the paper from the front withhammers arranged in a line across said sheet, the blow of each hammerbeing timed to be coincident with the passing of a character position bythe desired character for that position, the several hammers beingvariously timed to effect the printing along said print line of apredetermined line of copy within a single circulation of said carrier,the combination characterized by a. a plurality of such hammers, one foreach said column, each of said hammers comprising a rigid bar having ahead-end striking face adapted to press upon impact against the typeface of the selected character, said hammer being of a width closelyapproaching, but not exceeding, the width of said column, having flat,vertical, and parallel sides, and a cylindrical bottom surface, wherebysaid hammers when situated side-by-side have freedom to slide oneagainst the next, but are restrained by interference each with the nextagainst excessive tipping, each said hammer having a tail end adapted toabut the throwing arm of a corresponding hammer actuator, b. mountingmeans situated at the ends of said module, c. an equal plurality ofactuators, d. a hammer and actuator guide block, said block having endparts attached to said mounting means and a central portion, saidcentral portion having a face, situated facing and generally parallel tosaid sheet, parallel to said print line, a back generally parallel tosaid face, a top, extenDing generally perpendicular to said face andsubstantially coplanar with said print line, the upper surface of saidtop being figured by a sequence of parallel rounded grooves extendingperpendicular to said sheet and spaced on centers by the width of saidcolumns to support and space said hammers, and said block having a footwith toes extending outward from said block below and generally parallelto said grooves and centered between them, parallel and vertical sidesof said toes providing guiding slots for said actuators, said toeshaving bearing openings to support a pivot pin for all said actuators,e. a pivot pin passing through said openings, f. each of said actuatorscomprising a piece of metal having uniform thickness to fit between saidtoes having a depth sufficient to be substantially rigid in rotationabout said pivot pin, having a throwing arm portion tapering from saidpivot pin to a thin end to abut the tail end of said hammers, passingbetween said toes in said slots and pivotally mounted on said pivot pin,g. said actuators comprising a set of straight actuators and a set ofoffset actuators, said offset actuators being alternately installedbetween said straight actuators, each said straight actuator having astraight armature arm extending generally parallel to said sheet belowsaid pin as an extension of said throwing arm, having a soft-magneticarmature piece fixed to a side thereof, h. each of said offset actuatorshaving an offset armature extending generally perpendicular to saidsheet and perpendicular to said straight armature arms, having asoftmagnetic armature piece fixed to a side thereof, i. a firstelectromagnetic assembly comprising a plurality of electromagnets eachhaving a core and a magnetizing winding on said core, said cores beingassembled parallel, spaced apart on centers by twice said column widthand presenting a first working surface in which pole faces of said coresare regularly arrayed, j. a second electromagnet assembly substantiallyidentical to said first electromagnet assembly having a second workingface, k. said first electromagnet assembly being attached to saidmounting means to situate said first working face in close opposingrelationship to said armature arms of said straight actuators, each oneof said first surface pole faces being in operable opposition to acorresponding one of said armature pieces, and l. said secondelectromagnet assembly being attached to said mounting means to situatesaid second working surface in close opposing relationship to saidoffset armature arms, each one of said second surface pole faces beingin operable opposition to a corresponding one of said armature pieces.2. A printing mechanism comprising a. a hammer module as defined byclaim 1 wherein said mounting means comprise a pair of end framesgenerally parallel and extending away from said sheet and downwardlyfrom said line of text to a machine frame and b. a type-carrying drumrotatably supported on said end frames and continuously rotatable behindsaid sheet containing for each of said columns a font of type arrangedin a ring around said drum.
 3. A hammer module as defined by claim 1wherein said metal is mild steel.
 4. A hammer module as defined by claim3 in further combination with a flat spring piece cut to form a hammerreturn spring comb having teeth in the form of leaf springs each adaptedto engage at its tip the tail end of a corresponding one of saidhammers, the body of said piece being fastened to said back above saidtoes.
 5. A printer comprising a. a hammer module as defined by claim 4wherein said mounting means comprise a pair of end frames generallyparallel and extending away from said sheet and downwardly from saidline of copy to a machine frame and b. a type-carrying drum rotatablysupported on said end frames and continuously rotatable behind saidsheet containing for each of said columns a font of type arranged in aring around said drum.
 6. A hammer module as defined by claim 1 whereinsaid grooves are divided by a slot cut into said block top parallel tosaid print line into front and back portions, in further combinationwith a spring piece cut to form a hammer return spring comb having teethin the form of spring leaves each said leaf adapted to engage at its tipthe tail end of a corresponding one of said hammers, the body of saidpiece being fastened to said back above said toes, and wherein each ofsaid hammers has a recess at its tail end to engage one of said leaves.7. A hammer module as defined by claim 6 wherein said metal is mildsteel.
 8. A hammer module as claimed 1 in further combination with astraight stop bar maintained by said mounting means parallel to saidprint line in opposition to stop abutments on said straight actuators,whereby to establish the rest position of said actuators.
 9. A hammermodule as defined by claim 8 wherein said metal is mild steel.
 10. Ahammer module as defined by claim 9 in further combination with a springpiece cut to form a return spring comb having upward extending teeth inthe form of spring leaves each said leaf being adapted to engage arecess in the tail end of a corresponding one of said hammers, andhaving downward extending teeth situated to engage return springabutments on corresponding ones of said throwing arms, the body of saidpiece being fastened to said back above said toes.